Symmetric Key Encryption

Jonathan Katz, University of Maryland

Introduction

Symmetric Key Encryption Versus Public Key Encryption

Basic Notation and Definitions

Some “Classical” Encryption Schemes and Their Weaknesses

“Perfect” Secrecy and Its Limitations

Achieving Perfect Secrecy

Limitations of Perfect Secrecy

Computational Notions of Security

Beating the One-Time Pad

A Stronger Notion of Security

Pseudorandom Functions and Security Against Chosen-Plaintext Attacks

Symmetric Key Encryption in Practice

Block Ciphers and Stream Ciphers

Modes of Encryption

Hybrid Encryption

Stronger Definitions of Security

Further Information

Glossary

Cross References

References

INTRODUCTION

Symmetric key encryption schemes (also variously known as secret key, private key, or shared key encryption schemes for reasons that will become clear in a moment) allow users who have previously agreed on a shared, secret key to ensure the secrecy of their communication. A prototypical example might be two soldiers who wish to communicate securely while they are in the battlefield. Before heading to the battlefield (say, while they are together on base), these two soldiers can generate and share a random key k, which they will keep secret from everyone else. Later, when they are in the battlefield, these soldiers can use the common key k they have shared to communicate securely. In particular, when one soldier (the “sender”) wishes to send a message M (sometimes also called the plaintext) to the other (the “receiver”), ...

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